1. Field of the Invention
The invention relates to detecting certain types of signals in messaging systems, and more particularly to a system and a method for detecting telecommunication signals for the deaf in messaging systems.
2. Description of the Related Art
Presently, signals from telecommunication devices for the deaf (TDD) can be sent via regular telephone lines. These TDD signals can be directed to currently available voice messaging systems which store the sound of the TDD modem transmission. This sound can later be used to create a text display with the TDD message. Unfortunately, sound recordings of the TDD modem transmission use a lot of disk storage space in the receiving device's memory. For example, with a 16 kbps speech compression rate voice message system, a 30 second TDD transmission can take up to 60 k bytes to store a message. In comparison, using the common TDD transmission rate of 45.5 baud, a 30 second TDD transmission contains no more than 182 characters. Thus, storage of the characters rather than the sound recordings requires much less disk storage space. Moreover, if multiple forwarding is needed for a sound recorded TDD message, the signal quality of the sound recording is degraded by additional line distortion and multiple steps of speech compression. Even when forwarding is not involved, simply recording and playing back a TDD message leads to line degradation which occurs twice. If both of the lines are of poor quality, the resultant signal may be received incorrectly.
Alternatively, different telephone access numbers can be used for TDD signals and voice signals. In this arrangement, calls to the TDD access number are stored in TDD mode (usually as text), and calls to the voice access number are stored in voice mode (as sound recordings). In another arrangement, an additional signal, such as a dual-tone multi-frequency (DTMF) tone, is used to indicate to the messaging system that a TDD recording is desired. This additional signal is undesirable because some TDDs, particularly those built into computers, cannot generate the necessary signal. Thus, in these arrangements, the user sending the TDD signal must notify the receiving system about the TDD transmission before or when the TDD signal is sent. In systems that use different telephone access numbers, no mixed mode recording can be done. Additionally, more administration is needed to manage additional access numbers. Also, this type of system must be set up in advance and multiple access numbers issued.
Automated discriminators discriminate between signals from TDD and voice signals. These discriminators have several drawbacks. For example, if a TDD text transmission is received by a messaging system, the discriminator must quickly determine if the signal is a TDD signal. When the determination is not made fast enough, some of the TDD transmitted characters are lost. In addition, a human voice can mimic the sound of a TDD modem for a brief period of time. Such a voice signal can easily fool a fast discriminator into switching to the TDD mode when a voice recording is desired.
What is needed is a system which avoids losing TDD data during any TDD data transmission while also reliably storing voice data as sound recordings. It is also desirable to reduce the storage requirements for a TDD message stored by a messaging system which can also store voice messages. Furthermore, it is desirable to have a system which does not require the user to first notify the system that a TDD data transmission will be sent. Also, additional access numbers are undesirable. Finally, it is desirable to have a system and a method which have a minimal impact to the user interface of a regular messaging system.